The present invention relates to a hydrogen generator, and especially to a compact structure for the hydrogen generator.
Hydrogen plays an important role in the chemical process industries. The amount of hydrogen used in oil hardening, petroleum reforming, and ammonia producing processes, etc. is considerable. Consequently, engineers are still working on cheaper and more efficient methods of producing hydrogen, which are the goals of this invention.
Except from water electrolysis, pure hydrogen cannot be produced in a single step. However, electrolyzing water is only economically feasible using very cheap electricity. A general method is to first produce 'syngas' which is a mixture consisting of hydrogen, carbon monoxide, and carbon dioxide in different ratios. Syngas can then be further purified to obtain pure hydrogen, or directly used in places where hydrogen is needed.
To generate syngas, various types of hydrogen generators can be used. The structure of a common type of hydrogen generator is shown in FIG. 1. This type of hydrogen generator uses methane as its feed. When methane and water vapor are introduced into a catalyst bed 2 through an inlet 3, a steam reforming reaction begins, thus converting the feed into hydrogen and carbon monoxide. Combustible gas is introduced through a lower entrance 4 into the combustion chamber 1. After combustion, exhaust gas flows through the interior of the hydrogen generator so as to provide the heat required for steam reforming reactions by exchanging heat with the feed, thus improving the thermal efficiency.
However, this type of hydrogen generator still suffers from the following problems: The whole catalyst bed 2 is always at a high temperature. Since the water-gas shift reaction represented by equation (1) is an exothermic reaction, the product of the reaction contains a high percentage of toxic carbon monoxide at high temperature. ##STR1##
Consequently, the product cannot be used unless the carbon monoxide produced is converted into carbon dioxide by a water-gas shift reaction at a lower temperature in another reactor. As a result, the complexity of the hydrogen generator increases significantly.
In addition, since water cannot be directly used as a feed in this type of hydrogen generator due to its nature of destroying the catalyst bed, this type of hydrogen generator needs a source of steam. This creates additional problems in designing the entire system of the hydrogen generator.
FIG. 2 illustrates another type of hydrogen generator using methyl alcohol as feed. The mixture of methyl alcohol and water enters an evaporator 5, and evaporates therein. Then, the mixture in the gaseous state is guided into the tubular catalyst bed 6 and is converted into hydrogen and carbon monoxide through a methanol decomposition reaction shown by equation (2). Subsequently, carbon monoxide is further converted into carbon dioxide through the water-gas shift reaction mentioned before. ##STR2##
A combustion chamber 7 is built at the upper end of the hydrogen generator. Combustible gas enters and burns inside the combustion chamber 7, thus providing heat needed for the evaporating process and the steam reforming reaction. The evaporator 5 is positioned in the center of the generator so as to reduce heat losses.
From FIG. 2, it can be seen that the evaporator 5 takes up a considerable space in the generator. In comparison, a catalyst can only be placed in a limited volume within a tube having an annular section. Accordingly, the overall volumetric efficiency is poor.